I can load xml file at localhost but not in production server

Question

Until today, I was able to read daily exchange rates form Turkish Central Bank xml url. However, today, customers informed us that they cannot make online transaction on our website.Then I checked and realized that I can not read the contents of xml file. I am using php for that. Here is the code:

 $tcmb = ("https://www.tcmb.gov.tr/kurlar/today.xml"); 
 if (false === $tcmb) {
     echo "Failed loading XML\n";
     foreach(libxml_get_errors() as $error) {
         echo "\t", $error->message;
     }
 } 
 else { 
 $euro_satis = $tcmb->Currency[3]->BanknoteSelling;
 }

This returned with a "failed to load external entity" error.

Then I tried curl in order to make sure if file is closed to external reach.

 $ch = curl_init();
 curl_setopt($ch,CURLOPT_URL,"https://www.tcmb.gov.tr/kurlar/today.xml"); 
 curl_setopt($ch,CURLOPT_RETURNTRANSFER,true); 
 $output = curl_exec($ch); 
 curl_close($ch);
 $tcmb = simplexml_load_string($output);
 $euro_satis = $tcmb->Currency[3]->BanknoteSelling;

This returned null.

Then i tried both of the codes on local with Xampp (Apache Server) and both worked fine.

I tried to download the xml file from url. Again, it worked on local but not on production server. They all were working fine till today.

What could be wrong?